Nuclear Track Detectors (NTD) have been used in the detection of heavy charged particles for many years. Their main advantage, apart from low cost and ease of use, is the existence of natural thresholds of registration, which provide a natural and easy way of suppressing the background in an experiment looking for rare events (e.g. Strangelets, Monopoles) in cosmic rays and particle accelerators. In this presentation, I will talk about how an inexpensive, commercially available plastic called Polyethylene Terephthalate (PET), commonly used as a packaging and bottling material, was found to be suitable as a NTD with a threshold much higher than many other materials commonly used as NTDs and the use of PET in the search for strangelets in cosmic rays. In addition, I will describe how NTDs are being used in the search for monopoles at LHC.